An electret is a dielectric body having a high degree of surface electric charge. The electret retains its surface electric charge for a long period of time, often measured in tens of years. The electric charge, or dielectric polarization, is achieved, for example, in one prior art process by heating a body of dielectric material to a relatively high temperature while the material is exposed to an intense electric field for a substantial period of time, and then cooling the material while maintaining its exposure to the electric field. Upon cooling, the material exhibits characteristics of a permanent charge distribution.
Electrets have many uses as a transducer in a device such as a microphone, a loudspeaker and a record disk pick-up. Moreover, it is anticipated that electrets will be used in the near future as memory units, high potential sources, measuring apparatus and the like.
Electrets comprise dielectrics which produce static electric fields having relatively long lifetimes. Their long lifetimes are primarily attributable to the development of new materials which are more stable than previously employed wax materials. Today, plastic materials such as polyethylene, polypropylene, and polyethylene terephthalate are extensively used in constructing electrets.
Conventionally, electrets are constructed in the form of electret devices in which an electret body is secured on a conductive electrode. Such electret devices have drawbacks, in that their surface charge distribution is less than uniform.
Other electret devices having improved uniformity of surface charge distribution are described in co-pending U.S. patent applications, Ser. Nos. 280,056, 280,052 and 280,016 each filed June 30, 1981, and each entitled "ELECTRET DEVICE," by Tanaka and Wada.